Fixed bitrot in the non-softdep case of ufs_dirremove():

• restored async mount support. The first entry in a block is still always written synchronously, although it probably shouldn't be in the async case.
• restored use of BWRITE() instead of bowrite() for the DOWHITEOUT case, although bowrite() is probably better.

Broken by: merge of softdep changes (rev.1.22).
Found by: lmbench2 delete-file benchmarks.